The present invention is directed to a locked bone fixation assembly, and in particular to an assembly that allows for a surgeon-selected angle of the bone screw relative to the fixation device.
Orthopedic fixation devices, both internal and external, are frequently coupled to bone by the use of fasteners such as screws, threaded bolts or pins. For example, bone plates can be secured to bone with bone screws, inserted through plate holes. Securing the screws to the plate provides a fixed angle relationship between the plate and screw and reduces the incidence of loosening. One method of securing the screw to the plate involves the use of so-called “expansion-head screws.” U.S. Pat. No. 4,484,570 discloses an expansion-head screw with a head that has a recess, the walls of which contain a number of slits. After the expansion-head screw is inserted into bone through a hole in the fixation device, a locking screw is inserted into the recess to expand the walls of the recess to thereby lock the screw to the fixation device (such as a plate, internal fixator, nail, or rod). Another method of securing the screw to the plate involves the use of conical heads as shown in U.S. Pat. No. 5,053,036, which discloses conical screw holes, adapted to receive screws having conical heads of a predetermined cone angle, such that the plate will not slide down the heads of the screws. A third method of securing the screw to the plate involves the use of so-called “locking screws.” A locking screw has threading on an outer surface of its head that matches with corresponding threading on the surface of a plate hole to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known.
In addition to securing the screw to the fixation device, it is also often desirable to insert the screws at an angle relative to the fixation device selected by the surgeon. The prior art discloses a number of these so-called “polyaxial” systems, most of which utilize a bushing located in a hole in the fixation device to provide for locking at different degrees of angulation of the screw relative to the fixation device. For example, U.S. Pat. No. 5,954,722 discloses a polyaxial (selected variable axis) locking plate that includes a plate hole having a bushing rotatable within the hole. As a screw is being inserted into bone through the bushing and plate hole, a threaded tapered head of the screw engages a threaded internal surface of the bushing to expand the bushing against the wall of the plate hole, thereby friction locking the screw at the desired angular orientation with respect to the plate. U.S. Pat. No 6,575,975 discloses a polyaxial locking plate that includes a plate hole, having a bushing rotatable within the hole, a fastening screw and a locking screw. The head of the fastening screw includes a radial wall that allows for outward expansion so that outwardly expanding the sidewall of the bushing so that the fastening screw is locked to the bushing and fixation device.
Some others of the so-called “polyaxial” systems utilize a ring located in a hole in the fixation device. For example, U.S. Pat. No 6,454,769 discloses a plate system and method of fixation comprising a bone plate, a bone screw and a ring, said ring being expandable against the bone plate to fix the bone screw at a selected angle relative to the bone plate.
These multi-component traditional plate assemblies can be cumbersome and tedious to manipulate during surgery to achieve the most desirable angle for directing the bone screw into the patient.
The present invention relates to an improved locked bone fixation assembly that allows for a surgeon-selected angle of a bone screw relative to the fixation device in only one single surgical action and using only two components, plate and screws, so that no rings, bushing or expansion head screws are longer needed.